It is well recognized that silicon heterojunction bipolar transistors (HBTs) can be made to have advantageous properties of low noise and wide frequency response. This has been found particularly true of HBTs in which the silicon-germanium alloy base has a relatively high germanium fraction, i.e., about 30% or more, and a relatively high base doping level, i.e., about 5.times.10.sup.19 cm.sup.-3 or more. (Devices of this kind are described, e.g., in A. Schueppen, et al., IEDM Tech. Digest (1994) p. 377.) However, these devices are conventionally fabricated using blanket epitaxy on unpatterned substrates, followed by mesa isolation. Procedures of this kind are not compatible with advanced semiconductor fabrication processes that call for device structures having a greater degree of planarity.